Essential Molecular Biology (BIOC0007) Notes - DNA
3 views 0 purchase
Course
Essential Molecular Biology (BIOC0007)
Institution
University College London (UCL)
Explore Essential Molecular Biology at UCL. Unveil the intricacies of DNA dynamics, covering damaged DNA, repair mechanisms, detection, and cutting-edge recombinant technologies. Tailored for Year 1 students, these notes offer a humanized academic guide, enriching your understanding of DNA replicat...
DNA
DNA Structure and Packaging
DNA molecule
o Genetic code
Instructions that enable us to: live + reproduce + pass information to the next generations
Identical in almost all species – number of genes varies
Gene = linear string of DNA nucleotides – contains information to produce proteins in the
cell
Packaging
o Chromatin = open DNA
o Chromosome = packaging tightly DNA
o Nucleotides
Monomer of DNA
Structure
5 carbon pentose sugar + phosphate + nitrogenous base
+ +
Base
GUArdian Angels are PURe and TWO wINGed (gUAnine and Adenine are PURines
and TWO rINGed)
Purines = 2 ringed
o Adenine – A
o Guanine – G
Pyrimidines – 1 ringed
o Cytosine – C
o Thymine – T
o Uracil – U
RNA vs DNA
DNA = deoxyribose sugar
RNA = ribose sugar
DNA = thymine
RNA = uracil
o Nucleosides vs nucleotides
Nucleoside = sugar + base
Nucleotide = sugar + base + phosphate
Glycosidic bond = base
covalently bonded to sugar
o Between C1 sugar and
N9 purine / N1
pyrimidine
Phosphate ester bond = phosphate covalently bonded to nucleoside
o Phosphate attaches to the 3’ or 5’ OH of nucleoside
,DNA
o Polynucleotide
Linear sequence of nucleotides
Backbone = sugar + phosphate
Different bases
Phosphodiester bond
Nucleotides joined by phosphodiester bond
Directionality = 5’-3’
DNA double helix 3D conformation
o Two single strands associated via covalent hydrogen and van der Waals bonds – forming double
stranded DNA (dsDNA)
o Complementary base pairing
A + T = 2 hydrogen bonds
C + G = 3 hydrogen bonds
o Chargaff’s rule = ratio of purines to pyrimidines = 1:1
o Antiparallel strands
5’ end of one strand pairs with 3’ end of other
o Complementary strands
Physical properties of double helix
o Beta form
Most energetically favourable form of DNA
Two strands wind around one another in a right-handed double helix – every 10.5bp
o Helical structure repeats every 10.5bp
Sugar phosphate backbone is hydrophilic = lies outside of the helix (periphery)
Base pairs are hydrophobic = lies inside the helix (core) – base stacking
Base stacks bind asymmetrically with the sugar phosphate backbone – forming a
major (13Å) and minor (9Å) groove
DNA conformations
o Beta-DNA
Most abundant
Right-handed
Repeat unit = 10.5bp
Helix diameter = 20A
Central core = solid
Major groove = wide and deep
Minor groove = narrow and deep
o Alpha-DNA – results from DNA dehydration
Less common
Right-handed
Repeat unit – 11bp
Helix diameter – 26A
Central core – hollow
Major groove – narrow and deep
Minor groove – wide and shallow
o Z-DNA – results from methylation, stress, and high salt concentration
Least observed
Left-handed, zig-zag pattern
Repeat unit – 12bp
Helix diameter – 18A
Central core – solid
Major groove – flat
Minor groove – narrow and deep
, DNA
DNA folding into chromatin
o Histones
Nuclear proteins responsible for compacting and organising DNA
o Chromatin
Complex of histones + DNA = ½ DNA + ½ protein
o Euchromatin
Chromatin in relatively decondensed regions stained lightly + is transcriptionally active =
euchromatin
Found during interphase – when genome is open and active
o Heterochromatin
More compacted regions stain more darkly – transcriptionally more inactive =
heterochromatin
Found in periphery of nucleus during interphase + centromeres and telomeres of
chromosomes
Nucleosome
o 4 core histones – highly conserved
H2A + H2B + H3 + H4
Rich in lysine + arginine
Carry positive charges – counteract DNAs negative
charge = stabilising DNA
Each histone contains intrinsically disordered flexible N terminus
histone tail
19-39 residues
o Extend from the nucleosome complex
o Needed for chromatin condensation (1030nm)
o Influence chromatin structure
N-terminus tails = H2A, H2B, H3, H4
C-terminus tails = H2A, H2B
o 146bp of DNA wraps around a histone complex in a left-handed manner
= forming nucleosome
Nucleosome = unit of eukaryotic chromatin
o Histone octamer
Disk-shaped protein complex at the centre of the nucleosome
Contains 2 copies of each of the 4 core histones
Two monomers of H3 and H4 = form a tetramer
Followed by association of 2x H2A-H2B heterodimers
Levels of chromatin condensation
o Linker DNA
Nucleosomes – connected by free DNA = linker DNA
o Chromatin packaging has several layers – leading to more condensed DNA
1 – 10nm fibre = nucleosome association with linker DNA
2 – 30nm fibre = H1 linker histone bound to linker DNA nucleosomes move closer to each
other (beads on a string)
H1 is removed during DNA transcription
o Further condensation to form chromosomes
30nm fibre further condensed
Large loops of chromatin in 30nm fibres – anchored to central scaffold forming
chromosome
o Summary of condensation – 30nm 300nm 700nm 1400nm metaphase chromosomes
The benefits of buying summaries with Stuvia:
Guaranteed quality through customer reviews
Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.
Quick and easy check-out
You can quickly pay through credit card or Stuvia-credit for the summaries. There is no membership needed.
Focus on what matters
Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!
Frequently asked questions
What do I get when I buy this document?
You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.
Satisfaction guarantee: how does it work?
Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.
Who am I buying these notes from?
Stuvia is a marketplace, so you are not buying this document from us, but from seller sujansathiendran. Stuvia facilitates payment to the seller.
Will I be stuck with a subscription?
No, you only buy these notes for $8.49. You're not tied to anything after your purchase.
